Portion of a mask



FIG. 1 is a front perspective view of a portion of a mask according to an embodiment of our design;

FIG. 2 is a front view thereof;

FIG. 3 rear view thereof;

FIG. 4 is a right side view thereof, with the left side view being a mirror image thereof; and,

FIG. 5 is a top view thereof.

Where included, broken lines showing environment or structure are for illustrative purposes only and form no part of the claimed design.

Where included, dotted lines represent the border between claimed and unclaimed subject matter and form no part of the claimed design. 

The ornamental design for a portion of a mask, as shown and described. 